One of the world's largest and most knowledgeable scientific bodies wants to make one point very clear: Just as smoking causes cancer, so too are humanity's greenhouse gas emissions causing the planet to change, with potentially unknown and unalterable impacts.

The evidence is overwhelming. Temperatures are going up. Springs are arriving earlier. Ice sheets are melting. Seas are rising. Rainfall and drought patterns are changing. Heat waves are getting worse, as is extreme precipitation. The oceans are acidifying.

"The science linking human activities to climate change is analogous to the science linking smoking to lung and cardiovascular diseases. Physicians, cardiovascular scientists, public health experts and others all agree smoking causes cancer," the AAAS wrote in its report, "What We Know."

"And this consensus among the health community has convinced most Americans that the health risks from smoking are real. A similar consensus now exists among climate scientists, a consensus that maintains climate change is happening, and human activity is the cause."

Move the debate
Speaking to reporters at a teleconference, AAAS chief executive officer Alan Leshner said: "What we are trying to do is to move the debate from whether human-induced climate change is reality. We want to move the debate to: Exactly what should you do about it?"

"We are trying to provide a voice for the scientific community on this issue so that we can help this country, help the world, move this issue forward," he added.

The report also warns of a "small but real" chance that a warming climate will cause sudden and possibly unalterable changes to the planet.

This echoes the words used in the AAAS' 2007 report by the Intergovernmental Panel on Climate Change (IPCC), which said climate change might bring "abrupt and irreversible" impacts.

The significance of Tuesday's report lies not in its findings, which cover familiar ground, but in who is saying it: the world's largest general scientific body, and one of its most respected.

Headline message
The report's headline messages are unambiguous. It says climate change is occurring here and now: "Based on well-established evidence, about 97 percent of climate scientists have concluded that human-caused climate change is happening."

This agreement, the report continued, is documented not just by a single study, but by a converging stream of evidence over the past two decades from surveys of scientists, content analyses of peer-reviewed studies, and public statements issued by virtually every membership organization of experts in this field.

"We are at risk of pushing our climate system toward abrupt, unpredictable, and potentially irreversible changes with highly damaging impacts," the association concluded. "Disturbingly, scientists do not know how much warming is required to trigger such changes to the climate system."

"As emissions continue and warming increases, the risk increases".

Unprecedented speed
The AAAS says there is scarcely any precedent for the speed at which this is happening: "The rate of climate change now may be as fast as any extended warming period over the past 65 million years, and it is projected to accelerate in the coming decades."

Historically rare extreme weather like once-in-a-century floods, droughts and heat waves could become almost annual occurrences, it says. Antarctic and Greenland ice sheets could see large-scale collapse, the Gulf Stream could alter its course, the Amazon rain forest and coral reefs could die off, and mass extinctions could threaten ecosystems.

The authors acknowledge that what the AAAS is doing is unusual: "As scientists, it is not our role to tell people what they should do or must believe about the rising threat of climate change," the authors said.

"But we consider it to be our responsibility as professionals to ensure, to the best of our ability, that people understand what we know: human-caused climate change is happening."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.